1. Field of the Invention
The present invention relates to an apparatus for detecting whether or not wafers are exactly mounted as previously determined, on a wafer carrier.
2. Description of the Related Art
Semiconductor wafers mounted on a wafer carrier are usually carried from one place where a process is conducted to another place where another process is done in the course of manufacturing semiconductor integrated circuits. Each of these wafers, for example 26 pieces, is mounted on the wafer carrier, partly inserted in a pair of grooves on the wafer carrier which are formed at a predetermined interval in the longitudinal direction of the wafer carrier. The wafer counter is used in the course of manufacturing semiconductor integrated circuits to check up every lot of wafer carriers, for the number of wafers mounted on each wafer carrier and the presence or absence of a wafer in each pair of the grooves.
Japanese Utility Model Disclosure No. Sho 61-129340 proposes a wafer counter of this type. This wafer counter has light emitting and receiving elements which are intermittently moved in the longitudinal direction of the wafer carrier at such a pitch that is same as the pitch of the wafers arranged on the wafer carrier, and the optical axis of infrared rays between these light emitting and receiving elements is inclined to the axis of the wafers arranged on the wafer carrier which are to be located between the light emitting and receiving elements. When the infrared rays are shielded by a wafer and the quantity of light received by the light receiving element is thus reduced, it is detected that the wafer is present in its corresponding grooves on the wafer carrier and when the quantity of light is not reduced, it is detected that no wafer is present in the grooves. This process of detecting the presence or absence of a wafer is intermittently carried out at the same pitch as that of wafers arranged on the wafer carrier to detect the number of wafers mounted on the wafer carrier.
In the case of this conventional device, however, there is a fear that no exact detection can be attained relating to the number of wafers mounted on the wafer carrier and a position of a pair of grooves where wafer is absent, due to a miss-detection of the light receiving element resulting from fouling of wafers in the course of processing wafers. The conventional device can detect how many wafers are not present on the wafer carrier, but it cannot exactly detect the state of each of them when two wafers inserted in a pair of grooves and when one of wafers seats tilting in its corresponding grooves.
Japanese Utility Model Disclosure No. Sho 61-127639 proposes another device capable of eliminating the above-mentioned drawbacks. This device includes a reflector type photosensor for detecting wafers, a light-emitting element, a light-receiving element, and a timing plate located between these elements and having apertures spaced apart at the same intervals are the grooves cut in the wafer carrier. The reflector type photosensor and the light emitting and receiving elements are synchronized to move in the longitudinal direction of the wafer carrier and the timing plate. When an aperture of the timing plate is located between the light emitting and receiving elements, the light receiving element detects light emitted from the light emitting element and a timing signal is outputted responsive to this detection of light. The reflector type photosensor detects light reflected from the wafer and a wafer detection signal corresponding to the detected light is outputted. If the portion of the timing signal corresponding to the time when the light receiving element detects light coincides with the portion of the wafer detection signal corresponding to the time when the photosensor detects light reflected from the wafer, it will be discriminated that the wafer is present in that grooves in the wafer carrier which corresponds to these portions of the timing and wafer detection signals. When they do not coincide with each other, it will be discriminated that no wafer is present in the grooves or that the wafer is not seated rightly in the grooves even when it is present there.
The phase of the timing signal must completely coincide with that of the wafer detection signal in order to enable the device to fulfill its correct detection. It is therefore needed that the timing plate is correctly positioned.
However, positioning error of the timing plate is likely to be caused because the timing plate is mechanically fixed and its apertures are also mechanically formed. Further, the timing plate must be exchanged every time when the pitch of grooves on the wafer carrier changes. It takes a long time for the timing plate to be exchanged with a new one and for the new one to be adjusted upon its being fixed, thereby causing productivity to be lowered.